Anyone know what this is for? (random reinforced concrete under lawn)

Sorry for the vague title I wasn't sure how else to describe this or where to put it in the forum.

Anyhow i'm trying to clear my backyard of all the old turf and came across reinforced concrete just below some of the turf, by just below I mean the turf was laid directly over it! Anyhow I got bored of digging and using my fork this concrete leads all the way to my garage, see the picture below...



Does anyone have any idea what purpose this could possibly serve?

cheers, Lee


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Anyone know what this is for? (random reinforced concrete under lawn)